Corrosion resistant boiler tube for chemical recovery vapor generating unit

ABSTRACT

The disclosure relates to a corrosion resistant boiler tube for chemical recovery vapor generating units, in which a boiler tube of carbon steel or low alloy Cr-M. steel has weld deposited on its exterior surface a coating of high chromium steel containing 13-26 percent Cr and 0.5- 1.5 percent Cb.

United States Patent [72] inventors Tsuneto Hayashi,

Hidejiro Kinoshita; Koji lwahashi, Nagasaki-shi, Japan Appl. No. 790,182

Filed Sept. 26, 1968 Divlsi'm of Ser. No. 671,075, Sept. 27,

[54] CORROSION RESISTANT BOILER TUBE FOR CHEMICAL RECOVERY VAPOR GENERATING UNIT 3 Claims, 8 Drawing Figs.

[52] U.S.Cl 219/76 (ii) ecr-imo STEEL E.

[51] Int. Cl 823k 9/04 [50] Field of Search 219/76, 145; ll7/202-207 [56] References Cited UNITED STATES PATENTS 2,050,043 4/1936 De Golyer 219/76 2,164,072 6/1939 Moses et al. 219/76 3,185,814 5/1965 Rossner et a1 219/76 3,476,909 11/1969 Kameda et a1. 219/145 Primary Examiner.l. V. Truhe Assistant Examiner-Lawrence A. Rouse AttorneyMcGlew and Toren ABSTRACT: The disclosure relates to a corrosion resistant boiler tube for chemical recovery vapor generating units, in which a boiler tube of carbon steel or low alloy Cr-M. steel has weld deposited on its exterior surface a coating of high chromium steel containing 13-26 percent Cr and 0.51.5 percent Cb.

1i) STB 35 (iii) user-am STEEL iv l user-10 WELD 0F OVERLAID TEST TEMPERATURE (C PATENTEU JAN 1 2 l9" SHEEY 2 BF 3 PATENIEI] JAN I 2 I97! I TENSILE YIELD POINT STRENGTHM B) a S) Kg/mm REDUCTION Z' OF AREA Kg /mm 100 90 8O 7O 6O 011 010 oogootpcp SHEET 3 BF 3 FIG. 4

RT 360 460 560 660 730 S00 TEST TEMPERATURE G) I STB 35 CARBON STEEL Iecr-Icb STEEL zecr-IcbsTEEL 13cr-1cb TEEL TEST TEMPERATURE (C) INVENTOR. 'TSUNETO H RYR 3H1 H1 5 1RD KIN HIT K833 IwnH s I-II Q fl'w flhu/ Z T ue/r0 :ATTORNEYS CORROSION RESISTANT BOILER TUBE FOR CHEMICAL RECOVERY VAPOR GENERATING UNIT This is a division of application Ser. No. 67 l ,075, filed Sept.

BACKGROUND OF THE INVENTION ln the production of paper pulp, useful chemicals are recovered and steam power requirements are supplied, at least in part, by burning the waste liquor, used for digestion of the pulp, in a chemical recovery vapor generating unit. For example, such a unit may be used to recover soda smelt as well as to generate steam for the production of electric current and for the steam requirements of the pulp production plant. ln the chemical recovery unit or boiler, the organic ingredients of the waste liquor, such as lignin and sugars, are burned. During combustion, the inorganic components, such as principally sodium carbonate and sodium sulfate, are melted and the resulting soda smelt is recovered at the base of the combustion chamber of the recovery unit.

Recently, plants for the production of pulp have been of larger size and, because of this larger size, the chemical recovery units have been designed to operate at higher temperatures and under higher pressures. When the recovery unit is operated under the more severe conditions of higher temperature in the walls of the boiler tubes located in the combustion chamber, in the super heater, etc., the temperature can reach 350 C. or more. Under these conditions, severe difficulties are encountered in that the tubes are severely corroded due to the attack by soda smelt.

With boiler tubes of carbon steel or low alloy Cr-Mo steel, corrosion due to the attack by soda smelt occurs in those tubes which are located in the combustion chamber at and near to the inlet for the introduction of primary air, which is the location of highest temperature. Such corrosion also occurs preferentially in tubes located in the lower bent portion of the super heater. In some cases of severe corrosion, the rate of corrosion of the boiler tubes is of the order of 1 mm. per year in a unit operating under pressure of 100 kg./cm. As the wall thickness of a boiler tube in the combustion chamber is normally from 5 to 6 mm., the useful life of boiler tubes in the combustion chamber is usually about 4 to 5 years.

From the foregoing, it will be noted that corrosion usually occurs in tubes positioned in limited zones of the chemical recovery unit. Accordingly, if the tubes used in such limited zones can be protected by using a material having good resistance to attack by soda smelt, the durable life of the tubes may be increased considerably and it would be possible to construct a chemical recovery unit having a greater overall efficiency. Various method have been proposed to protect the walls of the tubes located in such limited zones. In one method, a number of studs are welded onto the tube wall and the interspaces between the studs are filled with an anticorrosive material resistant to the attack of soda smelt. In another method or process, an anticorrosive material is sprayed onto the tube walls. In still a further method or process which has been proposed, a protecting material of stainless steel is secured to the tube wall.

However, the first process mentioned above has the drawback that the studs corroded and reduced in length in a relatively short time, and to the extent that the anticorrosive material applied in the interspaced is liable to fall out. In the second process mentioned above, wherein anticorrosive material is sprayed onto the tube walls, this material is adhered only mechanically to the tube walls, and there is therefore the disadvantage that the anticorrosive material has a tendency to strip off from the tube walls. With the third process mentioned above, there is the hazard that the tube can be cracked owing to the difference in thermal expansions between the protecting material of stainless steel and the base material of the tube, if the protectingmaterial is inadequately secured to the base material of the tube. This third method has the further disadvantage, among others, that the heat efficiency of the recovery unit is reduced.

SUMMARY OF THE INVENTION This invention is directed to boiler tubes for use in chemical recovery vapor generating unitsand, more particularly; to such boiler tubes having improved resistance to corrosion and suitable, for example, fo'r'use in a chemical recovery vapor generating unit used to recover soda smelt?" I In accordance with the invention, a boiler tube, for use in a chemical recovery vapor generating unit, is provided in which the aforementioned disadvantages are avoided. Thus, in accordance with the invention, and inexpensive high Cr steel containing Cr and Cb is weld deposited on the exterior surface of the wall of a boiler tube of carbon'st eel or low alloy C r-Mo steel. The weld deposited material riot only has physical properties similar to those of the basem'aterial of thetube, but also has mechanical properties superior to those of the base material. Thus, the weld deposited material hasa substantially higher resistance to attack by soda smelt than does the base material, and moreover can be readily welded to the base material.

In particular, the material weld deposited on a base material of carbon steel or low alloy Cr-Mo steel comprises a high chromium steel containing 13-26 percent Cr and 0.5--l.5 percent Cb. A boiler tube in accordance with the present invention has a remarkably high resistance to corrosion, due to the particular coating weld deposited on the surface of the base material. With the boiler tube of the present invention, the anticorrosive deposited layer can neither drop off nor strip off the base material, because the weld deposited material has weldability so good that the weld deposited layer can-be adhered strongly and closely to the base material of theboiler tube. This is furthermore due to the fact that the weld deposited material has physical properties very similar to those of the base material of the boiler tube. An additional advantage is that the weld deposited material, in the case of a boiler tube embodying the present invention, is less expensive than Cr-Ni stainless steel. Consequently, the overall expense of the boiler tube is reduced while the boiler tube still has a long useful life since it exhibits a combination of the abovementioned advantages and also has a high resistance to corrosion due to soda smelt.

Boiler tubes embodying the present invention may be formed by weld depositing high Cr steel, containing a specific proportion of Cb, onto the base material by a hand welding method orby s a submerged arc welding method. The weld depositing of the protective material onto the base material is effected at those portions of the surface of the basic tube where corrosion is likely to occur due to attack by soda smelt. In order to provide an appropriately long life for the tube, it is necessary that the thickness of the weld deposited layer should be l mm or more. In practice, it is preferred, from the standpoint of economics, that the thickness of the weld deposited layer be up to 5 mm.

Each tube may have a coating weld deposited thereon while i it is still in the form of a single tube, the distortion of the tube may then be corrected by suitable measures, a number of the coated tubes may thereafter be assembled in a panel, and the assembly may then be annealed to eliminate stress in the assembly. Alternatively, the weld depositing of the coating may be effected after a group of tubes have been assembled into a panel.

The high Cr steel used as the weld deposited protective coating in the tube of the present invention should contain the specific proportions of Cr and Cb, because the percentages of Cr and Cb in the high Cr steel are critical as will be made apparent hereinafter.

Accordingly, an object of the present invention is to provide an improved corrosion resistant tube for use in waste recovery vapor generating units.

Another object of the invention is to provide such a tube comprising a base material having a high Cr steel weld deposited on its exterior surface.

A further object of the invention is to provide such a tube in which the base material is carbon steel or low alloy Cr-Mo steel and the protective coating weld deposited thereon is a high Cr steel including a specific proportion of C b.

Still another object ofthc invention is to provide such a tube formed of carbon steel or low allow Cr-Mo steel and having weld deposited on its outer surface a coating of high chromium-steel containing l-3-26 percent Cr and 0.5-1.5 percent Cb.

A further object of the invention is to provide such a boiler tube which is particularly resistant to corrosion due to attack by soda smelt,

Yet, another object of the invention is to provide such a tube including a base material having weld deposited thereon a corrosion resistant material, and in which the corrosion resistant material has physical properties similar to those of the base material but corrosion resisting properties very substantially in excess of those of the base material.

BRIEF DESCRIPTION OF THE DRAWINGS For an understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

In the drawing: 1

FIG. I is a graph which shows a relation between the Cr content of Cr steel and the rate of corrosion of the Cr steel due to the attack of soda smelt;

FIGS. 20 and 2b are microphotographic views of the deposition welded layer of 18 Cr-l Cb steel on the boiler tube of the present invention and of the weld deposited layer of 18 Cr steel;

FIG. 3 is a graph which shows the results of tests determining the rates of corrosion of various steel materials weld deposited on the surface of the boiler tube;

FIGS. 4 and 5 are graphs showing the physical and mechanical properties of various steel materials to be weld deposited on the surface of the boiler tube; and

FIGS. 6A, and Bare photographic views, respectively, of-the surfaces of the boiler tube of the present invention and of the surface of a prior art boiler tube.

DESCRIPTION OF THE'PREFERRED EMBODIMENTS Refen'ing toFlG. l which graphically illustrates the relation between the Cr content and the rate of corrosion due to attack by soda smelt, it may be seen that the anticorrosive properties of a Cr steel increase remarkably when the Cr content is 13 percent or greater. It appears that this high anticorrosive property is due to the passive state which will be developed in the surface of the Cr steel. Accordingly, the lower limit of Cr content should be l3 percent. Further increase of the Cr content beyond 26 percent is not economical, and it does not result in any substantial further improvement in the anticorrosive properties. Sometimes, it actually can produce coarser grains and deteriorate the mechanical properties of the Cr steel. Consequently, the upper limit of the Cr content, in accordance with the present invention, should be 26 percent.-

The presence of Cb makes the grains of the Cr steel finer,

and therefore results in improving the mechanical properties of the Cr steel and particularly the elongation and contraction of area of the Cr steel tube. The proportion of Cb in the high Cr steel should be within the range of 0.5 to 1.5 percent in accordance with the present invention, because the content of Cb less than 0.5 percent is not effective in making the grains of the Cr steel finer and because a content of Cb more than 1.5 percent is neither economical nor can it produce any further improvement in the mechanical properties of the Cr steel.

Referring to FIG. 2a, which is a microphotographic view of an 18 Cr-l Cb steel as the weld deposited material in accordance with the invention, and referring to FIG. 2b, which is a microphotographic view of an 18 Cr steel used as a reference material, it will be seen that the grains in the Cbcontaining Cr steel are very much finer than those in the reference Cr steel. These microphotographic views are magnified times. The boiler tube embodying the present invention will now be illustrated with reference to some embodiments thereof.

The corrosion-resistance, physical properties and mechanical properties of the boiler tube in accordance with the present invention and made by weld depositing l8 Cr-l Cb high Cr steel on the surface of a base material, comprising a steel tube of carbon steel STB 35 were tested, in comparison with the same properties of the base material of a boiler tube of carbon steel STB 35, of another boiler tube of 9 Cr-l Mo steel, and of a prior art protecting material comprising 18 Cr-] Ni stainless steel. The compositions of the carbon steel STB 35 and the 18 Cr1 Cb steel, used in the test, are tabulated in Table 1 below:

TABLE 1 Ingredients in percent The curves illustrated graphically in FIG. 3 show the results of tests determining the rate of corrosion of the different steel materials position in a solidified soda smelt at different temperatures. Curve I represents the results obtained with a tube formed of carbon steel STB 35, Curve ll represents the results obtained with a tube of 9 Cr-l Mo steel, Curve III represents the results obtained with a protecting material of 18 Cr-8 Ni stainless steel, and Curve IV represents the results obtained with a tube formed of carbon steel STB 35 having weld deposited on its outer surface 18 Cr-l Cb steel in accordance with the present invention. As will be seen from a comparison of these curves, the resistance to smelt corrosion of a tube having a coating of 18 Crl Cb weld deposited thereon, in accordance with the invention, is 12 times higher than that of the tube of carbon steel STB 35 and that of the tube of 9 Cr-l Mo steel at a relatively moderate test temperature of the order of 350 C.

The difference in the corrosion-resistance of a holder tube in accordance with the present invention from the corrosionresistance of reference materials is greater as the test temperature is increased. The corrosion-resistance resistance of a boiler tube formed in accordance with the present invention is not much different from that of a reference boiler tube of 18 Cr-8 Ni stainless steel at test temperatures in the vicinity of 350 c., but at higher temperatures, the resistance of the reference boiler tube of 18 Cr-8 Ni stainless steel is substantially lower than that of a boiler tube of the present invention. Furthermore, it has been found that a 13 Cr-l Cb steel, which may be used as the weld deposited corrosion-resistance coating within the scope of the present invention, shows a more or less lower resistance to smelt corrosion than does the 18 Cr-l Cb steel, but its corrosion-resistance may still be sufficient for certain uses within the scope of the present invention. The coating of 26 Cr-l Cb steel has a further improved resistance to smelt corrosion as compared to that of the 18 Cr-l Cb steel.

The thermal expansion coefficient and the thermal conductivity of conventional carbon steel STB 35, as well as those of 18 Cr-l Cb steel, 13 Cr-l Cb steel and 26 Cr-l Cb steel, used as the weld deposit coating in accordance with the present invention, are tabulated in Table 2:

TABLE 2 Thermal Thermal conduc- Temperexpansion tivity, ature eoellieient cal./em./

Steel in C.- X10- 6/ Carbon steel STB 35 100 11. 9 0. 122 300 13. 0 0. 110 500 14. 2 0. 094

18 Cr-l Cb steel 100 10. O 300 11. 0 0. 067 500 112. 0 0. 065

13 Cr-l Cb steel 100 9. 9 0. 06 300 10. 1 500 11. 5 0. 069

26 Cr-l Cb steel 100 10. 0 0. 052 300 11. 0

Table 2 shows that the average coefficient of thermal expansion of the 18 Cr-l Cb steel in a temperature range of 100 C. to 500 C. is ll.0 X [0- and is slightly smaller than the average value 13.0 X l0--") of the carbon steel STB 35 in the same range of temperature. Since the weld deposited layer of 18 Cr-l Cb steel on the surface of the boiler tube of the present invention, which contacts with the gases at higher temperature has a coefiicient of thermal expansion smaller than that of the base material of the tube, it is clear that the boiler tube of the invention can resist heat-stress with safety. Although the 13 Crl Cb steel has a slightly lower coefficient of thermal expansion than the 18 Cr-l Cb steel, the boiler tube with this 13 Cr-l Cb steel weld deposited thereoncan also resist heat-stress with safety. It may be appreciated that the 26 Cr-l Cb steel has a coefficient of thermal expansion,

smaller to that of the 18 Cr-l Cb steel.

Thermal conductivity of the 18 Cr-l Cb steel is somewhat lower than that of the carbon steel STB 35, but this does not produce trouble in the operation of the boiler tube of the present invention. Thermal conductivity of the 13 Crl Cb steel and of the 26 Cr-l Cb steel is substantially of the same order as that of the 18 Cr-l Cb steel and is better than that of 'l8 Cr-8 Ni stainless steel, though the table does now show photographic view in FIG. 6A. On the other hand, those tubes The 18 Cr-l Cb steel exhibits higher yield points and higher tensile strengths, but somewhat lower elongation and lower contraction of area, than the carbon steel STB 35, though it has a strength and toughness sufficient to be used as the weld deposited coating material according to the present invention. Both the 13 Cr-l Cb steel and .the 26 Cr-l Cb steel similarly have mechanical properties sufficient for use as the weld deposited material according to the present invention.

The 18 Cr steel, however, clearly cannot be used as the weld deposited coating material, as it has a very much lower elongation and contraction of area than does the 18 Cr-l Cb steel. it is apparent that the 18 Cr-l Cb steel exhibits a lower yield point than does the carbon steel STB 35 at higher temperatures, and that the 18 Cr-l Cb steel, as well as the 13 Cr-l Cb steel and 26 Cr-l Cb steel, show higher tensile strengths than does the carbon steel STB 35 at higher temperatures. It is seen that the 18 Cr-l Cb steel has a more or less lower contraction of area than the carbon steel STB 35 and that the 18 Cr-l Cb steel as well as the 13 Cr-l Cb steel and 26 Cr-l Cb steel exhibit somewhat lower elongations than the carbon steel STB 35. However, these Cb-containing steels still retain a toughness sufficient to be used as the weld deposited material with advantage.

When tubes embodying the present invention were used in an actually operating chemical recovery vapor generating unit, for recovering soda smelt, it was demonstrated practically that the durable life of the unit could be prolonged. Thus, when it was found that boiler tubes in the bottom portion of an operating unit for recovery of soda smelt had been corroded, and exhibited a dangerous state, some such corroded boiler tubes were repaired by weld depositing thereon a coating rod using a welding rod of mild steel, and others of the corroded boiler tubes were repaired by weld depositing thereon a coating using a welding rod of 18 Cr-l Cb steel. After the repaired boiler tubes were reinstalled and the unit operated for a further period of about 10 months, the tubes were removed from the unit and examined for surface conditions. It was found that those tubes having a coating of mild steel weld deposited thereon had been greatly corroded, as shown by the coated with the weld deposited layer of 18 Crl Cb steel did not have any substantial corrosion, as shown by the photothis. graphic view of FIG. 68.

TABLE 3 True Yield Tensile Contraction breaking Hard Treatment alter point, strength, Elongation, of area, load, abz Steel deposition-welding kg./mm. Ina/mm. percent percent percent Hv Carbon steel STB 35 Annealed at 900 22:3 gig No treatment 2 g 338 18 Cr-l Cb steel 3 3 29 6 Annealed at 625 4 5 4 No treatment 52. 6 65. 6 1. 0 18 "{Annealed at 625 0. 62.8 70. 2 3. 2 13 Or Cb steel..- Annealed at 625 0.--. 48. 8 63. 3 37. 0 26 Cr-l Cb steel do 40. 9 59. 4 26.

In Table 3, there are shown the mechanical properties of the conventional carbon steel STB 35 as well as of the 18 Cr-l Cb steel, the 13 Cr-l Cb steel and the 26 Cr-l Cb steel which are used as the weld deposited coating material for the boiler tube according to the present invention. The mechanical properties shown in Table 3 have been determined at normal temperature.

'The graph of FIG. 4 includes curves which represent the variations of tensile strength of the different steels at higher temperatures, as well as curves which represent the variations of yield point of the steels at higher temperatures.

The upper group of curves in the graph of FIG. 5 represents the variation of sinking-workability of the steels at higher temperatures, and the lower group of curves represents the variation of elongation of the steels at higher temperatures.

As previously mentioned, conventional tubes of carbon steel or low alloy Cr-Mo steel have usually been corroded, after only 3 to 4 years, to the extent that the wall thickness of the tubes has been reduced below the lower limit of the safety requirement of the vapor generator, and it is usual to replace conventional tubes, made of carbon steel or low alloy Cr-Mo steel, by new ones after 3 to 4 years. However, boiler tubes formed in accordance with the present invention have a durable life of several times to 10 times that of conventional boiler tubes, because the high chromium steel containing 13-26 percent Cr and 0.5-1.5 percent Cb deposited as a surface layer on the boiler tube is highly resistant to attack by soda smelt.

The high chromium steel, containing 13-26 percent Cr and 0.5l.5 percent Cb, weld deposited as the surface layer of boiler tubes in accordance with the invention exhibits physical properties very much similar to those of the carbon steel which is the base material of the tube, and moreover it has stronger mechanical properties than does the carbon steel. These properties of the Cr steel containing Cb, and having high resistance to corrosion, provide a tube formed in accordance with the invention with a toughness so high that a recovery unit can be operated at higher temperatures and pressures with greater efficiency than prior recovery units. Furthermore, when using a tube in accordance with the invention, it is not necessary to use an expensive Cr-Ni stainless steel. Accordingly, the tube in accordance with the invention has very substantial advantages from the economic standpoint.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

We claim:

1 A method of forming a boiler tube resistant to corrosion by soda smelt, when used as the tube of a chemical recovery vapor generating unit, said method comprising, forming a tubular body of a material selected from theigroup consisting of carbon steel and low alloy Cr-Mo steel; and fusion weld depositing, on the exterior surface of said tubular body, a high chromium steel consisting of l326 percent Crand 0.5-1.5 percent Cb, balance Fe and the usual impurities.

2. A method, as claimed in claim 1, in whichsaid high chromium steel is fusion weld deposited manually.

3. A method, as claimed in claim 1, in which said high chromium steel is fusion weld deposited by a submerged arc welding process. 

2. A method, as claimed in claim 1, in which said high chromium steel is fusion weld deposited manually.
 3. A method, as claimed in claim 1, in which said high chromium steel is fusion weld deposited by a submerged arc welding process. 